Wild mice from the Eastern Shore (ES) of Md. that fail to respond to phosphorylcholine (PC) immunization were mated to imbred mice and their progeny studied for 1) the immune response to PC; 2) the C3 and T15 allelic forms of anti PC antibodies described in inbred mice; and 3) the Igh-C allotypes. Wild mouse 378, phenotype PC-T15-C3 Igh-Ca was bred to C.B20 (PC+T15+C3+Igh-Cb) and then backcrosses (BC) were made to C.B20 and 378. Similar crosses of another (ES) mouse 645 (PC-T15-C3-Igh-Ca,e,f) which appeared to be an Igh-C recombinant were made. 56/59 F1 progeny of 378 and 645 bred to C.B20 failed to respond to PC, suggesting a dominant suppressor gene present in the wild mice. 49/51 progeny from BC to ES mice gave no PC response, suggesting control by at least 2 or more genes. 44/47 progeny from BC to C.B20 produced high antiresponses. The genetics of the expression of C3 and T15 allelic forms of anti PC antibodies appears to be more complex. F1 progeny frequently expressed low titers of C31d in mice with no anti PC antibody. Occasionally, T151d also appeared. In F1 mice BC to ES, a similar situation pertained, but less frequently. In x C.B20. C3 and T15 were present in the majority of the anti PC responder mice. There was no concordance of C3 and T15 expression and allotype.